The invention relates to a heating device for an energy storage device, in particular in a battery direct converter circuit for supplying power to electric machines, and to a method for heating energy cells of an energy storage device, in particular when starting an electrically driven vehicle.
The trend is that in the future electronic systems which combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as wind power installations or solar installations, and in vehicles, such as hybrid or electric vehicles.
FIG. 1 shows, by way of example, the feeding-in of alternating current into a three-phase electric machine 101. In this case, a DC voltage provided from a DC voltage intermediate circuit 103 is converted into a three-phase AC voltage by a converter in the form of a pulse-controlled inverter 102. The DC voltage intermediate circuit 103 is fed by a line 104 of battery modules 105 connected in series. In order to be able to meet the requirements for power and energy given for a respective application, a plurality of battery modules 105 is often connected in series in a traction battery 104. An energy storage system of this type is often used in electrically driven vehicles, for example.
The series circuit comprising a plurality of battery modules is associated with the problem that the entire line fails when a single battery module fails. Such a failure of the energy supply line can result in failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of an individual battery module can result in power reductions in the entire energy supply line.
Document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this type are known under the name multilevel cascaded inverter or else battery direct inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module lines, which can be connected directly to an electric machine or an electrical power supply system. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module lines in this case have a plurality of energy storage modules which are connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit, which makes it possible to interrupt the respective energy storage module line or to bypass the respectively associated at least one battery cell or to switch the respectively associated at least one battery cell into the respective energy storage module line, depending on control signals. By suitable driving of the coupling units, for example with the aid of pulse-width modulation, suitable phase signals for controlling the phase output voltage can also be provided, with the result that there is no need for a separate pulse-controlled inverter. The pulse-controlled inverter required for controlling the phase output voltage is therefore integrated in the BDI, so to speak.
BDIs generally have a higher degree of efficiency and higher degree of fail-safety in comparison with conventional systems, as shown in FIG. 1. The fail-safety is ensured, inter alia, by virtue of the fact that defective, failed or not fully effective battery cells can be disconnected from the energy supply lines by suitable bypass driving of the coupling units.
In the case of the two systems, both in the case of the system shown in FIG. 1 and in the case of a BDI, precisely when in use in electrically driven vehicles, it may happen that the battery cells are to be operated at low ambient temperatures, for example in winter. Conventional batteries, for instance lithium-ion batteries, have an internal resistance which is dependent on temperature. At low temperatures, the internal resistance is increased and so the full power cannot be taken from the battery under certain circumstances. Therefore, at low ambient temperatures, depending on the composition of the battery cells used, a preconditioning of the battery cells must take place, that is to say, the battery cells must be warmed so that the power required for driving, in particular for starting-up, can be provided. The heating-up process in this case is energy-intensive and time-consuming.